1. Field of the Invention
The present invention relates to high frequency signal generators and, more particularly, to a high frequency signal generator for frequency-multiplying a low frequency signal to a high frequency signal by using a magnetic-to-voltage multiplex characteristic of a superconducting quantum interference device.
2. Discussion of Related Art
Generally, as an apparatus for generating a high frequency wave such as a millimeter wave, a resonator having a variety of forms is connected to an active element such as a Gunn diode or a GaAs FET (field effect transistor) to generate a desired resonant frequency signal. The configuration and operation of such a high frequency signal generator will be described with reference to FIGS. 1, 2, 3A and 3B.
Referring now to FIG. 1, a resonant frequency is generated using a resonator 12 and an amplifier 11 consisting of FETs. Among signals of various frequency components generated from the amplifier 11, only a signal of a predetermined band passing the resonator 12 is continuously generated as a high frequency signal from an output terminal OUT through positive feedback.
Referring to FIG. 2, a Gunn diode 21 is used to generate the high frequency signal. Only a corresponding resonant frequency component generated from a resonator 22 among signals generated from the Gunn diode 21 is supplied to the exterior through an output terminal OUT.
FIG. 3A shows the operating principle of a high frequency signal generator using a superconducting junction. An AC effect is used indicating that if a constant voltage V is applied to a Josephson junction 30, a signal of a frequency component proportional thereto is generated. This can be. expressed by the following equation: ##EQU1## where Ic: superconducting critical current
.PHI.o: magnetic quantum=h/2e=2.0679.times.10.sup.-15 T-m.sup.2 PA1 e: electron charge=1.6.times.10.sup.-19 C PA1 h: Planck's constant=6.6262.times.10.sup.-34 Js PA1 V: DC voltage across Josephson junction, PA1 I: AC current flowing into Josephson junction, PA1 .phi.(o): constant phase, and PA1 f.sub.j : Josephson oscillation frequency by DC voltage.
FIG. 3B shows one example of a Josephson junction array using the above principle. If constant currents flow through a first high frequency choke coil 31A, bias lines 32, Josephson junctions 33 and a second high frequency chock coil 31B, a high frequency signal which varies linearly according to a voltage across the Josephson junction 33 is generated. If only one of the Josephson junctions 33 is used, since a power is weak, the above array structure is utilized to raise the output power.
However, the high frequency signal generator using the FET or the Gunn diode is difficult in actuality to generate a very high frequency such as a millimeter wave. Moreover, the high frequency signal generator using the AC effect of the superconducting Josephson junction also has difficulty in forming the Josephson junction under the conditions of high temperature superconductivity operated at a nitrogen temperature.